This was really my problem wayback I starting learning AJAX. I cannot figure it out on how to add AJAX capability if I already have an existing website. One solution I remember at that time is I created an AJAX enabled website and transfer all my work...
Creating Web Parts in ASP.NET 2.0 is one of the cooling thing I've ever seen in ASP.NET 2.0. I don't know if it was a new feature in 2.0 for ASP but it was nice development control for me. I think it was the same implementation with DotNetNuke...